When drilling in deep water (e.g., greater than 350 ft. water depth), a frequent practice is to employ a mobile offshore drilling unit (MODU). A typical MODU can be either a semi-submersible drilling unit or a drill ship. The common practice for conducting operations is to run a subsea blow out preventer (BOP), which uses an associated or contained apparatus called a ram to (i) close around various pipe diameters (pipe rams); (ii) fully close the well (blind rams); (iii) close around most pipe diameters (annular rams); or (iv) shear a pipe (shear ram). Such BOP system assemblies are usually either hydraulically and/or electrically actuated, with various controls disposed on the MODU, or even an acoustical activation that can be remotely activated away from the rig.
These BOPS are installed on a riser and positioned on the wellhead at or near the mud line. The riser is generally between 16-inches in diameter to 22-inches in diameter. The riser will usually also carry the control and activation lines from the MODU to the BOPs. The riser is typically tensioned and held in place by riser tensioners located on the MODU. Such systems have been used by MODUs for years in water depths over 10,000 ft.
Other systems, for example, as disclosed in U.S. Pat. No. 7,458,425 to Millheim et al. entitled System & Method of installing & Maintaining an Offshore Exploration & Production System Having an Adjustable Buoyancy Chamber show an adjustable buoyancy chamber disposed in communication with the riser underwater, so that riser tension can be varied to either raise the riser and/or a wellhead above the surface (e.g., for intervention) or lower the riser and wellhead (e.g., for safety during a storm, etc.) either with or without a drilling ship.
A common misconception associated with this configuration is that the riser/BOP assembly is sufficient to establish and maintain control of the well by closing it off in the event of a fluid incursion (whether oil, gas and/or water), and to maintain the integrity of the well by circulating incurred fluids in water in order to regain control of the well in an emergency.
However, a recent drilling accident in the Gulf of Mexico demonstrates that the traditional riser/BOP configuration can fail when hydrocarbons (oil and gas) are uncontrollably released from a well, thereby causing extensive environmental harm and loss of human life. Needless to say, the MODU used in the Gulf of Mexico incident was also destroyed. This event demonstrates that the industry's assumption about the existing BOP riser system, which was previously thought safe and nearly fool-proof, is incorrect and should be challenged.